The presently disclosed embodiments are directed to imaging members used in electrostatography. More particularly, the embodiments pertain to the preparation of flexible electrophotographic imaging members which have improved imaging layer(s) formulated to comprise of a plasticizer in a material matrix of a solid solution comprising a charge transporting compound and a film forming polymer binder which is a novel A-B diblock copolymer or a binary polymer blend of a novel A-B diblock copolymer and a bisphenol polycarbonate. The flexible imaging members thus prepared have improved photoelectrical cyclic function stability, chemical resistive property, and are curl-free, and thus eliminate the need for an additional anticurl back coating layer. The present disclosure relates to all types of flexible electrostatographic imaging members used in electrostatography.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Flexible electrostatographic imaging members are well known in the art. Typical electrostatographic imaging members include, for example: (1) electrophotographic imaging members (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging members for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. All the electrostatographic imaging members are prepared in either flexible belt form or rigid drum configuration and could either be a negatively charged or positively charged design.
For a typical flexible electrophotographic imaging member belt used in a negatively charged imaging system, the imaging member belt comprises a charge transport layer, a charge generating layer, and optional layers on one side of a supporting substrate layer and does also include an anticurl back coating on the opposite side of the substrate to imaging member flatness. In this flexible electrophotographic imaging member, where the charge generating layer is sandwiched between the top outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer should be capable of generating electron hole pair when exposed imagewise and inject only the holes through the charge transport layer. In the alternate case where the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer which also provided with an anticurl back coating to render imaging member flatness.
A typical flexible electrographic imaging member belt may, however, have a more simple material structure and include a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Alternatively, the electrostatographic imaging members can also be a rigid member, such as those utilizing a rigid substrate support drum. For these drum imaging members, having a thick and rigid cylindrical supporting substrate bearing the imaging layer(s), no application of an anticurl back coating layer is needed.
All the flexible electrostatographic imaging members may be seamless or seamed belts. Seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam.
Although the scope of the present embodiments covers the preparation of all types of electrostatographic imaging members in flexible belt design or rigid drum configuration, for reasons of simplicity, the discussion hereinafter will focus and be represented only by flexible electrophotographic imaging member belts of negatively charged design.
Typical and conventional negatively-charged electrophotographic imaging member belts, such as photoreceptor in conventional flexible belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, and a typical charge transport layer of about 29 micrometers in thickness. The charge transport layer is the thickest and usually the last layer, or the exposed outermost layer, to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing solution application of the charge transport layer coating and through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support. Since the charge transport layer in a typical electrophotographic imaging member device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature.
The exhibition of imaging member curling after completion of charge transport layer coating is due to the consequence of the heating/cooling processing step, according to the mechanism: (1) as the web stock carrying the wet applied charge transport layer is dried at elevated temperature, dimensional contraction does occur when the wet charge transport layer coating is losing its solvent during 120° C. elevated temperature drying, but at 120° C. the charge transport layer remains as a viscous flowing liquid after losing its solvent. Since its glass transition temperature (Tg) is typically between 85 and 90° C. (depending on the polymer binder used), the charge transport layer after loss of the solvent will re-adjust itself, release internal stress, and maintain its dimension stability; (2) as the charge transport layer now in the viscous liquid state is cooling down further and reaching its glass transition temperature (Tg), the charge transport layer instantaneously solidifies and adheres to the charge generating layer because it has then transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) eventual cooling down the solid charge transport layer of the imaging member web from its Tg down to 25° C. room ambient will then cause the charge transport layer to contract more than the substrate support since it has about 3 to 4 times greater thermal coefficient of dimensional contraction than that of the substrate support. This differential in dimensional contraction results in tension strain built-up in the charge transport layer which therefore, at this instant, pulls the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock (having a 29-micrometer charge transport layer and using a 3½ mil thick polyethylene terephthalate substrate) will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and renders the imaging member web stock with desired flatness.
Electrophotographic imaging member web upward curling is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. Moreover, imaging member belt curling affects electrical charging uniformity across the belt width, under photo-electrical machine belt function condition, causing copy printout quality degradation. An anticurl back coating, having an equal counter curling effect but in the opposite direction to the applied imaging layer(s), is applied to the reverse side of substrate support of the active imaging member to balance the curl caused by the mismatch of the thermal contraction coefficient between the substrate and the charge transport layer, resulting in greater charge transport layer dimensional shrinkage than that of the substrate. Although the application of an anticurl back coating is effective to counter and remove the curl, the resulting imaging member in flat configuration creates tension and an internal built-in strain in the charge transport layer of about 0.27 percent in the layer. The magnitude of charge transport layer internal built-in strain is very undesirable, because it is additive to the induced bending strain of an imaging member belt as the belt bends and flexes over each belt support roller during dynamic fatigue belt cyclic motion under a normal machine electrophotographic imaging function condition in the field. The summation of the internal strain and the cumulative fatigue bending strain sustained in the charge transport layer has been found to exacerbate the early onset of charge transport layer fatigue/flexing induced cracking, preventing the belt to reach its targeted functional imaging life. Moreover, imaging member belt employing an anticurl backing coating has additional total belt thickness to thereby increase charge transport layer bending strain and speed up belt cycling fatigue charge transport layer cracking. The cracks formed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print-out defects, which thereby adversely affect the image quality on the receiving paper.
Various belt function deficiencies have also been observed in the common anticurl hack coating formulations used in a typical conventional imaging member belt, such as the anticurl back coating does not always provide satisfying dynamic imaging member belt performance result under a normal machine functioning condition. For example, exhibition of anticurl back coating wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of a belt and requires its frequent costly replacement in the field. Anticurl back coating wear under the normal imaging member belt machine operational conditions reduces the anticurl back coating thickness, causing the loss of its ability to fully counteract the curl as reflected in exhibition of gradual imaging member belt curling up in the field. Curling is undesirable during imaging belt function because it leads to non-uniform charging. In addition, developer applicators and the like, during the electrophotographic imaging process, may all adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances can manifest as variations in high background deposits during development of electrostatic latent images near the edges of paper. Since the anticurl back coating is an outermost exposed backing layer and has high surface contact friction when it slides over the machine subsystems of belt support module, such as rollers, stationary belt guiding components, and backer bars, during dynamic belt cyclic function, these mechanical sliding interactions against the belt support module components not only exacerbate anticurl back coating wear, it does also cause the relatively rapid wearing away of the anti-curl produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Moreover, anticurl back coating abrasion/scratch damage also produces unbalance force generation between the charge transport layer and the anticurl back coating to cause micro belt ripples formation during electrophotographic imaging processes, resulting in streak line print defects in output copies which deleteriously impact image printout quality and shorten the imaging member belt functional life.
Moreover, high contact friction of the anticurl back coating against machine subsystems is further seen to cause the development of electrostatic charge built-up problem. Static charge built-up in anticurl back coating increases belt drive torque, in some instances, has also been found to result in absolute belt stalling. In other cases, the electrostatic charge build up in the anticurl back coating during dynamic imaging member belt cyclic motion can be so high as to cause electrical sparking.
Lastly, the inclusion of an anticurl back coating as an added coating layer contributes to manufacturing cost. Moreover, application of anticurl back coating requires the imaging member web to be unwound and re-sent through the coater to add the anticurl back coating layer and increases the chances that the charge transport layer will be damaged from extra handling, thus adding to the imaging member production yield loss.
To overcome all these shortcomings, several attempts to eliminate the need for an anticurl back coating have been pursued. One of the more successful anticurl back coating-free flexible imaging members was achieved by reducing the charge transport layer internal stress/strain build-up, through incorporation of a plasticizer in the layer to minimize/eliminate the tension pulling force and effect curl suppression. For example, U.S. Pat. No. 6,183,921 discloses a crack resistant and curl-free electrophotographic imaging member in which the charge transport layer is comprised of an active charge transporting polymeric tetraaryl-substituted biphenyldiamine and a plasticizer. U.S. Pat. No. 7,008,741; discloses an imaging member having the charge transport layer and an optional overcoat formulated with the used of cross-linking a liquid carbonate. The imaging electrostatographic member obtained exhibits improved service life. U.S. patent application Ser. No. 12/551,414 to Yu et al. discloses an imaging member having a charge transport layer comprising a mix of plasticizers. U.S. patent application Ser. Nos. 12/551,440 and 12/782,671, both to Yu et al., disclose an imaging member having a charge transport layer comprising a single plasticizer or at least a single plasticizer. U.S. patent application Ser. No. 12/663,698 to Yu et al. discloses an imaging member having a charge transport layer comprising a single plasticizer. U.S. patent application Ser. No. 12/633,698 to Tong et al. discloses an imaging members comprising fluoroketone and 12/726,207 to Yu et al. discloses curl-free imaging members with slippery surface.
Although the attempts described in all the above disclosures are encouraging, the results were limited since those imaging members were unable to yield an absolute imaging member flatness due to the limitation of plasticizer that can be incorporated into the charge transport layer material matrix for effecting total internal stress/strain relief and without negatively impacting the photo-electrical performance of the prepared imaging members. Because for plasticizer loading level exceeding 9 weight percent (based on the total weight of the charge transport layer formulated to comprise a polycarbonate, diamine charge transporting compound, and di-ethyl phthalate plasticizer) in the charge transport layer of a typical imaging member, electrical Ve cycle-up has been discovered to be an issue and impacts copy printout quality: the appearance of negative image ghosting defects became evident in the print copies after few thousand print volumes. Even though total elimination of the residual curling to effect reasonable imaging member flatness for the 9 weight percent loaded charge transport layer was alternatively achievable by using thicker substrate support for greater beam rigidity to resist curl, the increase in the total imaging member thickness had the undesirable consequence of increasing the imaging member belt surface bending stress/strain to exacerbate early onset of charge transport layer cracking under a normal machine belt fatigue cyclic function condition over each belt module support rollers in the field.
In addition, another print quality problem associated with the conventional negatively charged imaging member has also recently emerged in the field, which is the ghosting image copy print defect. Result of chemical analysis has determined that the root cause of xerographic image print defects lies on absorption of amine species on the surface of the imaging member since the pre-printed images are formed on these papers with the use of amine agents catalyzed ultraviolet (UV) cured ink prior to xerographic imaging formation, resulting in amine vapor impact on copy printout quality degradation. The deposition and accumulation of amine residues onto the imaging member charge transport layer surface, after repeatedly making contact with receiving papers during xerographic imaging process, is found to cause ghosting image defects print-out in the output copies. Since ghosting image defects in the output copies are unacceptable print quality failures, it requires frequent costly imaging member replacement in the field.
To overcome the limitation of anticurl back coating-free imaging member designs developed in recent years and to eliminate the pre-printed paper amine contaminant associated print defect issue described in the preceding, there exists a need for an improved curl-free imaging member design. To achieve this purpose, the improved charge transport layer(s) of the present embodiments: (a) gives better imaging member flatness outcome (b) allows incorporation of plasticizer in any suitable loading level in the charge transport layer material matrix to impart greater electrical stability and maximize curl control, and (c) also renders the charge transport layer with resistivity to amine species attack for effecting the resolution of current pre-printed paper ghosting image defects copy printout issue.
Conventional electrophotographic imaging members and photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the undercoat layers: U.S. Pat. No. 5,660,961; U.S. Pat. No. 5,215,839; and U.S. Pat. No. 5,958,638. The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.” The term “electrostatographic” includes “electrophotographic” and “xerographic.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.”